In the optical communication field, a simplest scheme has been used that effects an intensity modulation as signal modulation and converts optical intensity directly into an electrical signal using a light detector. In recent years, however, a method of effecting phase modulation as signal modulation is drawing attention in order to address high bit rates above 40 Gbps. There are two methods of demodulating a phase-modulated signal: a method of demodulating a light transmitted after undergoing signal modulation by interfering it with a light from a local oscillator provided in a receiver (the coherent method); and a method of splitting a signal-modulated light into two beams, combining and interfering them with one bit of modulated signal out of phase, and converting a phase shift into optical intensity signal (the differential phase shift keying method). The differential phase shift keying method is drawing attention as a method relatively closer to practical use, since it is not necessary to synchronize the frequency of a signal light with that of a locally emitted light, unlike the coherent method. The differential phase shift keying is called Differential Binary Phase Shift Keying (DBPSK or DPSK), Differential Quadrature Phase Shift Keying (DQPSK), or the like, depending on the number of phases to be modified.
The demodulation method of the DPSK is described below with reference to FIG. 1. A DPSK-modulated signal 101 enters a delay line interferometer 102, and is split into two beams by a splitter like a half beam splitter 103. One of the two split lights is given an optical path of 1 bit long (for example, if signal modulation frequency is 40 GHz, about 7.5 mm) with respect to the other split light by a delay part 104 consisting of mirrors, and is set so that an optical path length difference between the two split lights becomes an integral multiple of light wavelength (that is, phase difference is 0). Then, the two split lights are recombined by a half-beam splitter 105 and two interference lights 106, 107 are generated. At this time, since an interference light 106 undergoes a constructive interference when an amount of phase shift between adjacent bits is 0 and a destructive interference when it is π, a phase difference is converted to an intensity difference for the interference light according to a phase shift amount between the adjacent bits. Since an interference light 107 is an interference light whose phase differs by π from the interference light 106, destructive interference results when the interference light 106 undergoes constructive interference and constructive interference results when the interference light 107 undergoes destructive interference, and thus a reversed light intensity is output. A demodulated signal is obtained by detecting an intensity difference between these interference lights with a differential detector 110 consisting of a balanced photodetector 108 and a trans-impedance amplifier 109.
Demodulation in DQPSK is performed by using two interferometers similar to those used for demodulation in the DPSK, as shown in FIG. 2. More specifically, a modulated signal 200 is split into two beams by a half-beam splitter 201, respective split lights are led to different interferometer 202, 203, and two reference lights generated in the respective delay line interferometers are detected by differential detectors 204, 205. However, while the delay line interferometer 202 sets a delay part 206 so that an optical path length difference between the two split lights becomes an integral multiple, the delay line interferometer 203 sets a delay part 207 so that an optical path length difference between the two split lights differs by (n+¼)λ (n is an integer and λ is wavelength of a light). At this time, when an amount of phase shift between adjacent bits is 0, π, a constructive interference or a destructive interference occurs in the delay line interferometer 202, and when π/2, 3π/2, a constructive interference or a destructive interference occurs in the delay line interferometer 203. This makes it possible to demodulate four-valued differential phase shift keying signals from outputs of differential detectors 204, 205. Furthermore, it is possible to demodulate any M-valued differential phase shift keying signals.
As an implementation of abovementioned delay line interferometer, two configurations are conceived; one in which a planar lightwave circuit is mainly used and one in which a free space optics with bulk optical components is used. The former is easy to mass-produce, but has disadvantages such as high power consumption due to required temperature control, and large size. In contrast, the latter is easy to decrease power consumption and can be made relatively smaller in size, thus drawing attention as a dominant implementation.
Meanwhile, in the abovementioned delay line interferometer that demodulates a DPSK-modulated signal, the wavelength of the modulated signal is not constant, and generally the wavelength of an incoming light differs according to the configuration of a communication system. If the wavelength entering the delay line interferometer differs, a value set for optical path length difference naturally varies and therefore the delay line interferometer will require means for adjusting optical path length. Also, even if the wavelength is constant, in a case where optical path length drifts due to temperature changes, adjustment of the optical path length to offset the drift is required. As optical path length adjusting means like this, a method of using thermo-optic effect and a method of using a Piezo actuator are shown in Patent Document 1 (JP-A-2007-306371) and Patent Document 2 (JP-A-2008-537652) respectively. The method of using thermo-optic effect is a method in which a medium whose refractive index with high temperature dependence (dn/dT), such as silicon monocrystal, is inserted in the optical path of a delay line interferometer, and optical path length (phase difference) is controlled by controlling temperatures of this medium. The method of using a Piezo actuator is a method in which a mirror reflecting split lights in the delay line interferometer is attached to a Piezo actuator, and phase difference control is effected by changing the mirror position according to a drive voltage of the Piezo actuator.
Patent Documents 1 and 2 (JP-A Nos. 2007-306371 and 2008-537652) show delay line interferometers using a free space optics, but delay line interferometers using a planar lightwave circuit basically make optical phase modulation by means of temperature control. In Patent Document 3 (JP-A-2007-67955), for example, optical phase modulation is realized by causing changes in reflective index and thermal expansion of a wave guide and an effective change in optical path length of a optical path through the wave guide. Also, in Patent Document 3, a method of performing a optical phase modulation using electro-optic effect is described. In this case, optical phase modulation of a light is performed by driving a voltage for an electro-optic element to change its reflective index.
Furthermore, in Patent Documents 4 and 5 (JP-A Nos. 2007-082094 and 2007-043638) describe that relative phase difference is given by making asymmetric applied voltages and temperatures for phase shift components disposed in a set of planar lightwave circuit, such as electrodes, thin-film heaters, piezoelectric elements, etc.
Patent Documents 6 and 7 (JP-A Nos. 2004-286783 and WO04/099848) relate to a distributed slope compensation element and describe controlling a central wavelength of distribution, but their technical field, i.e., distribution slope compensation, differs from that of the present application.